1. Field of the Invention
This invention is directed to a digital electronic controller with an A.C. induction drive and a shaft encoder to replace control means on industrial machines, for example, industrial sewing machines.
2. Description of the Prior Art
Industrial sewing machines, dating 1950-60s, employ an A.C. induction, fractional horsepower motor and a clutch-brake drive to impart motion to the sewing head. Machines with drives of this vintage require the operator to manually modulate the clutch and brake to control sewing speed. The method for modulation is via a spring loaded lever arm coupled directly to the clutch-brake drive whereby the operator uses the side of the leg to push and release the lever arm with a lateral motion. Pushing the arm to the side releases the brake and engages the motor with the output shaft using the clutch. Releasing the arm disengages the clutch and applies a braking force. Admittedly low in its degree of speed regulation and high in frictional content, clutch-brake drives continue to be manufactured and used today because of low initial cost. This type of control system is referred to as a manually controlled clutch-brake drive.
With the development of integrated circuits and power transistors, an industrial controller was devised and built (1960-70s) which could accept the operator's speed command from the stroke of a foot pedal. No lever arm was incorporated to directly control the clutch and brake. Instead, the foot pedal mechanism is mechanically coupled to a position sensing device which is wired to the electronic controller. The stroke of the foot pedal is converted by the controller into two modulated output signals, each having opposite phase. Both output signals are wired to two solenoids in the clutch-brake housing. One signal energizes the clutch solenoid and the other energizes the brake solenoid. A longer stroke decreases the duty cycle of the brake signal and increases the duty cycle of the clutch signal, thereby increasing the sewing speed. A shorter stroke has the opposite effect whereby the sewing speed is reduced. The operator no longer uses the side of the leg to move a spring loaded lever arm, but rather uses the foot to control a pedal much like an accelerator pedal in a car. The foot pedal motion modulates the electronic signals that control solenoid action. The solenoids provide the force to engage or disengage the clutch-brake mechanism. This type of control system is referred to as an electronically controlled clutch-brake drive.
Motion controllers targeted for sewing machine applications have recently taken on a slightly different form. In 1986, Juki and Quick-Rotan announced production availability of a brushless D.C. motor (BLDC), electronic drive, and a digital controller programmed for sewing applications. The BLDC uses magnets on the rotor to interact with a rotating field to produce rotor torque. The electronic drive precisely controls the rotating field as a function of rotor position and command input to the drive. The BLDC is typically more expensive to manufacture than the A.C. induction motor, but the BLDC's response is linear and the electronic drive does not rely on friction as does the clutch-brake drive. This results in better response and less maintenance. But to retrofit a machine with this control system means to remove a good, low-cost A.C. induction motor and replace it with the BLDC motor.